Lithography is the process of printing from specially prepared surfaces, some areas of which are capable of accepting lithographic ink, whereas other areas, when moistened with water, will not accept the ink. The areas which accept ink form the printing image areas, generally hydrophobic areas, and the ink-rejecting areas form the background areas, generally hydrophilic areas.
In the art of photolithography, a photographic material is made imagewise receptive to oily or greasy inks in the photo-exposed (negative-working) or in the non-exposed areas (positive-working) on a hydrophilic background.
In the production of common lithographic printing plates, also called surface litho plates or planographic printing plates, a lithographic base that has affinity to water or obtains such affinity by chemical treatment is coated with a thin layer of a photosensitive composition. Compositions for that purpose include light-sensitive materials such as light-sensitive polymers, diazonium salts or resins, a photoconductive layer, a silver halide emulsion etc. These materials are then image-wise exposed to actinic radiation and processed in the appropriate manner so as to obtain a lithographic printing plate.
In another embodiment, a silver precipitating (nucleating) agent is located in or on top of the hydrophilic surface. An image is obtained on the precipitating layer according to the silver salt diffusion transfer process by contacting said precipitating layer with an exposed silver halide emulsion in the presence of a silver halide developing agent and a silver halide solvent.
According to another embodiment there is provided a heat mode imaging element for making a lithographic printing plate comprising on a lithographic base with a hydrophilic surface a first layer including a polymer, soluble in an aqueous alkaline solution and a top layer on the same side of the lithographic base as the first layer which top layer is IR-sensitive and unpenetrable for an alkaline developer wherein said first layer and said top layer may be one and the same layer.
A hydrophilic receiving layer can also be used for xerography, electrography, thermosublimation, thermal transfer and other image-forming techniques.
Several types of supports can be used for the manufacturing of a lithographic imaging printing plate. Common supports are for example organic resin supports, e.g. polyesters, and paper bases, e.g. polyolefin coated paper. These supports are first coated with a hydrophilic layer forming the hydrophilic lithographic background of the printing plate.
It is known to use as hydrophilic layer in these systems a layer containing polyvinyl alcohol and hydrolyzed tetra(m)ethyl orthosilicate and titanium dioxide and preferably also silicium dioxide as described in e.g. GB-P-1 419 512, FR-P-2 300 354, U.S. Pat. Nos. 3,971,660 and 4,284,705, EP-A-405,016 and 450 199.
To obtain superior physical properties, it is very important to control the drying process. Avoiding drying stress results in materials with superior physical qualities. In fact, drying stresses, caused by the pressure gradient in the liquid phase, result in the occurrence of microscopic fractures. The exterior of the gel shrinks much faster than the interior so that tensile stresses arise that tend to fracture the network at the exterior. These fractures propagate easily during the further drying process.
A number of measures can be taken to avoid these defects. To obtain good results, aging the gel before drying is recommended. However this is uneconomical and industrially not feasible.